Directional antenna
A directional antenna or beam antenna is an antenna which radiates greater power in one or more directions allowing for increased performance on transmit and receive and reduced interference from unwanted sources. Directional antennas like Yagi-Uda antennas provide increased performance over dipole antennas when a greater concentration of radiation in a certain direction is desired.
All practical antennas are at least somewhat directional, although usually only the direction in the plane parallel to the earth is considered, and practical antennas can easily be omnidirectional in one plane.
The most common types are the Yagi-Uda antenna, the log-periodic antenna, and the corner reflector, which are frequently combined and commercially sold as residential TV antennas. Cellular repeaters often make use of external directional antennas to give a far greater signal than can be obtained on a standard cell phone. Satellite Television receivers usually use parabolic antennas.
For long and medium wavelength frequencies, tower arrays are used in most cases as directional antennas.
Antenna typesIsotropic / · Isotropic radiator
·
Omnidirectional / · Batwing antenna
· Biconical antenna
· Cage aerial
· Choke ring antenna
· Coaxial antenna
· Crossed field antenna
· Dielectric resonator antenna
· Discone antenna
· Folded unipole antenna
· Franklin antenna
· Ground-plane antenna
· Halo antenna
· Helical antenna
· J-pole antenna
· Mast radiator
· Monopole antenna
· Random wire antenna
· Rubber ducky antenna
· Turnstile antenna
· T2FD antenna
· T-Antenna
· Umbrella antenna
· Whip antenna
·
Directional / · Adcock antenna
· AS-2259 Antenna
· AWX antenna
· Beverage antenna
· Cantenna
· Cassegrain antenna
· Collinear antenna array
· Conformal antenna
· Corner reflector antenna
· Curtain array
· Dipole antenna
· Doublet antenna
· Folded inverted conformal antenna
· Fractal antenna
· G5RV antenna
· Gizmotchy
· Helical antenna
· Horn antenna
· Horizontal curtain
· HRS antenna
· Inverted vee antenna
· Log-periodic antenna
· Loop antenna
· Microstrip antenna
· Offset dish antenna
· Patch antenna
· Phased array
· Parabolic antenna
· Plasma antenna
· Quad antenna
· Reflective array antenna
· Regenerative loop antenna
· Rhombic antenna
· Sector antenna
· Short backfire antenna
· Sloper antenna
· Slot antenna
· Sterba antenna
· Vivaldi antenna
· WokFi
· Yagi-Uda antenna
·
Application-specific / · ALLISS
· Evolved antenna
· Ground dipole
· Reconfigurable antenna
· Rectenna
· Reference a
Antenna Efficiency
The efficiency of an antenna relates the power delivered to the antenna and the power radiated or dissipated within the antenna. A high efficiency antenna has most of the power present at the antenna's input radiated away. A low efficiency antenna has most of the power absorbed as losses within the antenna, or reflected away due to impedance mismatch.
[Side Note: Antenna Impedance is discussed in a later section. Impedance Mismatch is simply power reflected from an antenna because it's impedance is not the correct value; hence, "impedance mismatch". ]
The losses associated within an antenna are typically the conduction losses (due to finite conductivity of the antenna) and dielectric losses (due to conduction within a dielectric which may be present within an antenna).
The antenna efficiency (or radiation efficiency) can be written as the ratio of the radiated power to the input power of the antenna:
/ [Equation 1]Efficiency is ultimately a ratio, giving a number between 0 and 1. Efficiency is very often quoted in terms of a percentage; for example, an efficiency of 0.5 is the same as 50%. Antenna efficiency is also frequently quoted in decibels (dB); an efficiency of 0.1 is 10% or (-10 dB), and an efficiency of 0.5 or 50% is -3 dB.
Equation [1] is sometimes referred to as the antenna's radiation efficiency. This distinguishes it from another sometimes-used term, called an antenna's "total efficiency". The total efficiency of an antenna is the radiation efficiency multiplied by the impedance mismatch loss of the antenna, when connected to a transmission line or receiver (radio or transmitter). This can be summarized in Equation [2], where is the antenna's total efficiency, is the antenna's loss due to impedance mismatch, and is the antenna's radiation efficiency.
/ [Equation 2]Since is always a number between 0 and 1, the total antenna efficiency is always less than the antenna's radiation efficiency. Said another way, the radiation efficiency is the same as the total antenna efficiency if there was no loss due to impedance mismatch.
Radiation Pattern
A radiation pattern defines the variation of the power radiated by an antenna as a function of the direction away from the antenna. This power variation as a function of the arrival angle is observed in the antenna's far field.As an example, consider the 3-dimensional radiation pattern in Figure 1, plotted in decibels (dB) .
Figure 1. Example radiation pattern for an Antenna (generated with FEKO software).
This is an example of a donut shaped or toroidal radiation pattern. In this case, along the z-axis, which would correspond to the radiation directly overhead the antenna, there is very little power transmitted. In the x-y plane (perpendicular to the z-axis), the radiation is maximum. These plots are useful for visualizing which directions the antenna radiates.
Typically, because it is simpler, the radiation patterns are plotted in 2-d. In this case, the patterns are given as "slices" through the 3d plane. The same pattern in Figure 1 is plotted in Figure 2. Standard spherical coordinates are used, where is the angle measured off the z-axis, and is the angle measured counterclockwise off the x-axis.
Figure 2. Two-dimensional Radiation Patterns.
If you're unfamiliar with radiation patterns or spherical coordinates, it may take a while to see that Figure 2 represents the same radiation pattern as shown in Figure 1. The radiation pattern on the left in Figure 2 is the elevation pattern, which represents the plot of the radiation pattern as a function of the angle measured off the z-axis (for a fixed azimuth angle). Observing Figure 1, we see that the radiation pattern is minimum at 0 and 180 degrees and becomes maximum broadside to the antenna (90 degrees off the z-axis). This corresponds to the plot on the left in Figure 2.The radiation pattern on the right in Figure 2 is the azimuthal plot. It is a function of the azimuthal angle for a fixed polar angle (90 degrees off the z-axis in this case). Since the radiation pattern in Figure 1 is symmetrical around the z-axis, this plot appears as a constant in Figure 2.
A pattern is "isotropic" if the radiation pattern is the same in all directions. Antennas with isotropic radiation patterns don't exist in practice, but are sometimes discussed as a means of comparison with real antennas.
Some antennas may also be described as "omnidirectional", which for an actual antenna means that the radiation pattern is isotropic in a single plane (as in Figure 1 above for the x-y plane, or the radiation pattern on the right in Figure 2). Examples of omnidirectional antennas include the dipole antenna and the slot antenna.
The third category of antennas are "directional", which do not have a symmetry in the radiation pattern. These antennas typically have a single peak direction in the radiation pattern; this is the direction where the bulk of the radiated power travels. These antennas are very common; examples of antennas with highly directional radiation patterns include the dish antenna and the slotted waveguide antenna. An example of a highly directional radiation pattern (from a dish antenna) is shown in Figure 3:
Figure 3. Directional Radiation Pattern for the Dish Antenna.
In summary, the radiation pattern is a plot which allows us to visualize where the antenna transmits or receives power.
In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the directional (angular) dependence of the strength of the radio waves from the antenna or other source.[1][2][3]
Particularly in the fields of fiber optics, lasers, and integrated optics, the term radiation pattern may also be used as a synonym for the near-field pattern or Fresnel pattern.[4] This refers to the positional dependence of the electromagnetic field in the near-field, or Fresnel region of the source. The near-field pattern is most commonly defined over a plane placed in front of the source, or over a cylindrical or spherical surface enclosing it.[1][4]
The far-field pattern of an antenna may be determined experimentally at an antenna range, or alternatively, the near-field pattern may be found using a near-field scanner, and the radiation pattern deduced from it by computation.[1] The far-field radiation pattern can also be calculated from the antenna shape by computer programs such as NEC. Other software, like HFSS can also compute the near field.
The far field radiation pattern may be represented graphically as a plot of one of a number of related variables, including; the field strength at a constant (large) radius (an amplitude pattern or field pattern), the power per unit solid angle (power pattern) and the directive gain. Very often, only the relative amplitude is plotted, normalized either to the amplitude on the antenna boresight, or to the total radiated power. The plotted quantity may be shown on a linear scale, or in dB. The plot is typically represented as a three-dimensional graph (as at right), or as separate graphs in the vertical plane and horizontal plane. This is often known as a polar diagram.
Contents
Fading
In wireless communications, fading is deviation of the attenuation affecting a signal over certain propagation media. The fading may vary with time, geographical position or radio frequency, and is often modeled as a random process. A fading channel is a communication channel comprising fading. In wireless systems, fading may either be due to multipath propagation, referred to as multipath induced fading, or due to shadowing from obstacles affecting the wave propagation, sometimes referred to as shadow fading.
Key concepts
The presence of reflectors in the environment surrounding a transmitter and receiver create multiple paths that a transmitted signal can traverse. As a result, the receiver sees the superposition of multiple copies of the transmitted signal, each traversing a different path. Each signal copy will experience differences in attenuation, delay and phase shift while travelling from the source to the receiver. This can result in either constructive or destructive interference, amplifying or attenuating the signal power seen at the receiver. Strong destructive interference is frequently referred to as a deep fade and may result in temporary failure of communication due to a severe drop in the channel signal-to-noise ratio.
A common example of deep fade is the experience of stopping at a traffic light and hearing an FM broadcast degenerate into static, while the signal is re-acquired if the vehicle moves only a fraction of a meter. The loss of the broadcast is caused by the vehicle stopping at a point where the signal experienced severe destructive interference. Cellular phones can also exhibit similar momentary fades.
Fading channel models are often used to model the effects of electromagnetic transmission of information over the air in cellular networks and broadcast communication. Fading channel models are also used in underwater acoustic communications to model the distortion caused by the water.
Slow versus fast fading
The terms slow and fast fading refer to the rate at which the magnitude and phase change imposed by the channel on the signal changes. The coherence time is a measure of the minimum time required for the magnitude change or phase change of the channel to become uncorrelated from its previous value.
· Slow fading arises when the coherence time of the channel is large relative to the delay constraint of the channel. In this regime, the amplitude and phase change imposed by the channel can be considered roughly constant over the period of use. Slow fading can be caused by events such as shadowing, where a large obstruction such as a hill or large building obscures the main signal path between the transmitter and the receiver. The received power change caused by shadowing is often modeled using a log-normal distribution with a standard deviation according to the log-distance path loss model.
· Fast fading occurs when the coherence time of the channel is small relative to the delay constraint of the channel. In this regime, the amplitude and phase change imposed by the channel varies considerably over the period of use.
In a fast-fading channel, the transmitter may take advantage of the variations in the channel conditions using time diversity to help increase robustness of the communication to a temporary deep fade. Although a deep fade may temporarily erase some of the information transmitted, use of an error-correcting code coupled with successfully transmitted bits during other time instances (interleaving) can allow for the erased bits to be recovered. In a slow-fading channel, it is not possible to use time diversity because the transmitter sees only a single realization of the channel within its delay constraint. A deep fade therefore lasts the entire duration of transmission and cannot be mitigated using coding.
The coherence time of the channel is related to a quantity known as the Doppler spread of the channel. When a user (or reflectors in its environment) is moving, the user's velocity causes a shift in the frequency of the signal transmitted along each signal path. This phenomenon is known as the Doppler shift. Signals traveling along different paths can have different Doppler shifts, corresponding to different rates of change in phase. The difference in Doppler shifts between different signal components contributing to a single fading channel tap is known as the Doppler spread. Channels with a large Doppler spread have signal components that are each changing independently in phase over time. Since fading depends on whether signal components add constructively or destructively, such channels have a very short coherence time.
In general, coherence time is inversely related to Doppler spread, typically expressed as